Ultrashort laser pulses enable fundamental studies on small length and time scales. Additionally, high pulse energies allows the access to new regimes of light matter interaction and the investigation of nanometer scale structures on attosecond time scales by XUV pulses produced via high harmonic generation (HHG). Unfortunately, the XUV photon flux is typically very low. Hence, high power and high repetition rate driving laser sources are required in order to improve the performance of current studies and to open the way for new exiting applications, such as seeding of free electron lasers. Regrettably, conventional (Ti:Sa) laser technology is limited in output power due to the thermo optical effects in the amplifier crystals. The objective of this thesis is the development of a new power scalable laser concept merging OPCPA technology with state-of-the-art high power fiber lasers. Based on modeling of the optical parametric amplifier, important requirements on the OPCPA pump are found which are adopted in choice and development of the pump laser later. Furthermore, the geometry of the optical parametric amplifier is optimized for ultra-broadband amplification. Gain narrowing and saturation effects are investigated in order to achieve high conversion efficiency. In addition, parasitic nonlinear effects, such as second harmonic generation of signal and idler wave, are studied and configurations are found which effectively avoid these unwanted effects. Experimentally, pulse durations of 8 fs and a pulse peak power as large as 6 GW are achieved with an optimized ultra-broadband OPCPA system. In addition, this few-cycle OPCPA system delivers an average output power as large as 6.7 W, which represents a record value for few-cycle lasers. Finally, high harmonic generation is demonstrated with this laser system and further scaling potential to higher peak and average powers is discussed.